Get the software (hopefully it remains a stable link in the future)
Reading the VirtualBox documentation it looks like you have to APT-GET a few programs before installing and some other changes to the Permissions.
sudo apt-get install dkms
I also have some problems installing this above and had to run the following per directions from terminal
sudo apt-get install -f
Once you find the correct version you need to download it. You will notice that it is a .deb package. To install the .deb package you need to run the following command:
sudo dpkg --install virtualbox-2.1_2.1.4-42893_Ubuntu_intrepid_amd64.deb
You might find that you have some problems but I hope that it install anyway. Now add yourself to the GROUP vboxusers in the System/Administration/Users and Groups
Now run this command and see if it will check the system and everything should be good after this
Now you might want to check the Applications Menu to see if it is completely install. Applications/System Tools.
Good luck and hope it works for you and you have fun.